Rechargeable-Battery Implantable Medical Device Having a Primary Battery Active During a Rechargeable-Battery Undervoltage Condition

An implantable medical device, comprising:

• a first battery for providing a first battery voltage;
• a second battery for providing a second battery voltage;
• a first battery voltage detector configured to determine whether the first battery is undervoltage;
• a first load;
• and
• a second load,
• wherein if the first battery is not undervoltage,
• the first load is powered by the first battery voltage, and
• the second load is powered by the first battery voltage, and
• wherein if the first battery is undervoltage,
• the first load is not powered, and
• the second load is powered by the second battery voltage or the first battery voltage.

An implantable medical device, comprising:

• a first battery for providing a first battery voltage;
• a second battery for providing a second battery voltage;
• a first battery voltage detector configured to compare the first battery voltage to a first threshold, and to produce at least one first control signal indicative of the comparison;
• at least one switch configured to selectively couple the first battery voltage to a first load in accordance with the at least one first control signal;
• and
• a power supply selector configured to set a power supply voltage to the first battery voltage or the second battery voltage in accordance with at least the at least one first control signal, wherein the power supply voltage powers a second load.